Automatic identification (“Auto-ID”) technology is used to help machines identify objects and capture data automatically. One of the earliest Auto-ID technologies was the bar code, which uses an alternating series of thin and wide hands that can be digitally interpreted by an optical scanner. This technology gained widespread adoption and near-universal acceptance with the designation of the Universal Product Code (“UPC”)—a standard governed by an industry-wide consortium called the Uniform Code Council. Formally adopted in 1973, the UPC is one of the most ubiquitous symbols present on virtually all manufactured goods today and has allowed for enormous efficiency in the tracking of goods through the manufacturing, supply, and distribution of various goods.
However, the bar code still requires manual interrogation by a human operator to scan each tagged object individually with a scanner. This is a line-of-sight process that has inherent limitations in speed and reliability. In addition, the UPC bar codes only allow for manufacturer and product type information to be encoded into the barcode, not the unique item's serial number. The bar code on one milk carton is the same as every other, making it impossible to count objects or individually check expiration dates, much less find one particular carton of many.
Currently, retail items are marked with barcode labels. These printed labels have over 40 “standard” layouts, can be mis-printed, smeared, mis-positioned and mis-labeled. In transit, these outer labels are often damaged or lost. Upon receipt, the pallets typically have to be broken-down and each case scanned into an enterprise system. Error rates at each point in the supply chain have been 4-18% thus creating a billion dollar inventory visibility problem. However, Radio Frequency Identification (RFID) allows the physical layer of actual goods to automatically be tied into software applications, to provide accurate tracking.
The emerging RFID technology employs a Radio Frequency (RF) wireless link and ultra-small embedded computer chips, to overcome these barcode limitations. RFID technology allows physical objects to be identified and tracked via these wireless “tags”. It functions like a bar code that communicates to the reader automatically without needing manual line-of-sight scanning or singulation of the objects.
A problem frequently encountered is that of “hot” tags. Because tags communicate with the reader by backscattering the carrier signal, those tags very close to a reader create a very strong backscatter that can interfere with communications between other tags and readers located far away. Two types of interference, or “jamming”, are prevalent: forward link jamming and backscatter jamming. Consider a situation in which passive tag-1 is located 0.5 meters from Reader #1. The communications therebetween include the forward link from the reader to tag, and the backscatter signal from the tag to the reader. The maximum effective range of Reader #1 is 10 m. Passive tag-2 is located 10 meters from Reader #2. Readers #1 and #2 are located 200 meters apart. A “hot” tag-1 located only 0.5 meter from Reader #1 will generate backscatter 400 times greater at 0.5 m than it would at the 10 m maximum range of the reader, as calculated by (max range/actual distance)2=(10/0.5)2=400×. As will soon become apparent, tag-1 generates so much backscatter that it can jam communications between tag-2 and Reader #2, even though both tag-2 and Reader #2 are located 200 meters away from tag-1. In fact, for acceptable communications, Reader #2 would need to be located over 600 meters from tag-1 and/or Reader #1 based on square-law attenuation of RF energy over distance in free space, as calculated by the following equation:D=a×b×(d2/d1)   Equation 1where:
D is the distance between Reader #1 and Reader #2,
a is the extra distance necessary to provide a minimum “tag-2 to tag-1 signal to noise ratio” of at least 10 db (which is a typical minimum ratio that allows tag-2 to successfully communicate with Reader #1) [value can vary depending on system and environmental situation],
b is the maximum effective range of Reader #1,
d2 is the distance between tag-2 and Reader #2, and
d1 is the distance between tag-1 and Reader #1.
Performing the calculation, 3×10×(10/0.5)=600 meters. This is unacceptable in situations where multiple readers may be present in close proximity, as in a shopping mall. In the US, there are about 50 channels available to RFID systems. In Europe, there are currently only 10 channels. Accordingly, as RFID becomes more prevalent, readers will be using the same channels and will be using the same frequency, and the “hot tag” problem will become a serious issue that must be overcome.
Using long-range Class-3 tags and readers makes this “hot tag” problem even worse. For example, a “hot” Class-3 tag-3 located 0.5 m from Reader #3 running at a full 4 Watt (W) Effective Incident Radiated Power (EIRP) power can jam a Class-3 tag-4 located 100 meters from Reader #4 at a range of 60,000 meters in free space, where D=3×100×(100/0.5)=60,000 meters. In English units, this “hot tag” can jam every reader operating in its channel at a range of up to 40 miles away (in free space).
One proposed solution is to have the tag detect its own incident power. If the tag detects a strong signal, it will attenuate its own backscatter. However, this adds complexity and cost to each and every tag, making it cost prohibitive.
What is needed is a cost effective and efficient way to both dramatically reduce the severity of the hot tag backscatter problem and also reduce reader-to-reader interference in the forward link as well.